Electric motor which converts an electric energy into a mechanical energy exhibits in general, due to an electromagnetic loss, such as the so-called iron loss by the hysteresis and eddy current in the iron core, and due to a mechanical loss by friction and vibration, not much better conversion efficiency. In order to maintain the motor operation, electric power enough to compensate such conversion loss should be supplied successively.
Heretofore, attempts have been proposed for increasing the conversion efficiency by, for example, improvement of the circuits included in the electric instruments and of the material of magnetic elements and by elimination or reduction of friction-causing portions, whereby nevertheless no satisfactory result has been attained.
One of the essential causes therefor resides in that there is a limitation in the characteristics of the permanent magnet indispensable for an electric rotary machine, such as motor or generator.
At present, the magnetic material exhibiting the highest magnetic energy product BH.sub.max (B=magnetic flux density, H=magnetic coercive force) is that of neodium iron magnet (Nd--Fe--B) which has a BH.sub.max value in the order of 36 MGOe. However, this magnetic material has a low Curie point and an inferior temperature characteristic of the magnetic flux density. This material is disadvantageous in the point that the coercive force is low, as seen from the demagnetization curve A of FIG. 1, in which the magnetic flux density decreases in proportion to the magnetic field strength. With such a magnetic material, a sufficient conversion efficiency cannot be achieved.
If the BH.sub.max value is high and the permeance coefficient is large, it means in itself that there is preserved a large amount of energy convertible into an electric energy or into a mechanical energy. Thus, if a magnetic material has a high BH.sub.max value combined with a large permeance coefficient, its magnetization becomes more difficult and a considerable amount of energy will be consumed for the magnetization, so that it can be assumed that a magnet with a high BH.sub.max value and a large permeance coefficient possesses in itself an amount of magnetic energy corresponding to that required for its magnetization.
Therefore, it would have been able to realize an electric motor or a power-generating motor which exhibits a very high energy efficiency, if the magnetic energy accumulated in a magnetic material as mentioned above was able to be utilized effectively. Thus, it was not a mere dream to realize a power-generative motor which can attain a long-term driving with a power regeneration by a lower energy input, if a permanent magnet having a sufficiently high BH.sub.max value combined with a large permeance coefficient (hereinafter denoted simply as a "high BH.sub.max permanent magnet") were provided. Nevertheless, such a magnet material had not been discovered.